/system-design-case-catalog

name: system-design-case-catalog
description: "Answer classic system design problems as constraint-to-solution sketches and coach interview practice: URL shortener, rate limiter, news feed, chat, notification, autocomplete, crawler, unique id. Use for interview practice or naming the closest known shape for a new problem."
metadata:
  triggers:
    keywords:
      - system design interview
      - design twitter
      - design url shortener
      - news feed
      - design chat system
      - web crawler
      - unique id generator
      - video streaming
      - video publishing
      - ride hailing
      - ride dispatch
      - payment ledger
      - payment timeout
      - flash sale
      - multi-tenant SaaS
      - cache stampede
      - notification outage
      - chat reconnect
      - database migration

Case Catalog

**Priority: P2 (MEDIUM)**

Each classic problem has one defining constraint. Name it first; the rest of the design follows.

Defining Constraints

| Problem | Defining constraint | Decisions that follow | | --- | --- | --- | | URL shortener | Read-heavy by ~100:1, key must be short and unique | Base62 of a distributed counter, cache-first read path, 301 vs 302 choice | | Rate limiter | Decision must be cheap, shared, and correct under concurrency | Token bucket in a shared counter, fail-open or fail-closed rule, `429` plus `Retry-After` | | News feed | Fan-out cost versus read latency, with celebrity skew | Push for normal accounts, pull for celebrities, hybrid merge at read | | Chat | Delivery guarantees and presence at persistent-connection scale | WebSocket gateways, per-conversation ordering, offline queue, read receipts | | Notification | Multi-channel delivery with retries and dedupe | Queue per channel, idempotency key, user preference and quiet hours | | Autocomplete | Sub-100ms prefix lookup over a huge term space | Trie or prefix index in memory, precomputed top-k per prefix, async rebuild | | Web crawler | Politeness and dedupe at scale, not raw fetching | Frontier queue per host, robots cache, URL fingerprint dedupe, freshness policy | | Unique id | Ordered, unique, generated without a central lock | Snowflake-style timestamp plus node plus sequence; clock-skew handling | | Video streaming | Bitrate ladder and CDN economics, not the upload | Transcode pipeline per rendition, adaptive manifests (HLS/DASH), edge cache hit ratio as the cost lever | | Ride hailing | Geo matching under moving supply and demand | Geohash or S2 cells, driver location stream with TTL, matching window and surge as a pricing signal | | Payment ledger | One ledger effect per business operation despite retries and partial failure | Stable business-operation identity, double-entry ledger, and reconciliation; see the [idempotency contract](../system-design-principles/references/idempotency-contract.md) |

Coaching Mode

• Mock rounds, the clock, the rubric, and the debrief live in `system-design-interview-coaching`; this catalog is its problem bank.
• Give the model answer only after the candidate commits to an approach; the defining constraint above is the follow-up question when they stall.
• For production practice, lazy-load [production case packs](references/production-case-packs.md). They are synthetic fixtures unless a named primary source says otherwise; never present their numbers or timelines as deployed facts.

Production Case Routing

• Choose one pack only after stating the profile, workload, SLO, team, budget, and invariant. Recalculate all numbers for the user's system.
• Carry the pack's selected views through the HLD-to-LLD trace; use only the view that answers the current question and do not create a second diagramming lane.

Reuse Rules

• Map a new problem to the nearest catalog shape, then re-derive the numbers. The shape transfers, the sizing never does.
• State where the analogy breaks before borrowing the design.
• A catalog answer is a starting hypothesis, not a substitute for intake and estimation.

Anti-Patterns

• **No pattern-matching without numbers**: a known shape still needs this system's QPS and data volume.
• **No interview answer as a build plan**: production adds migration, cost, compliance, and team constraints.
• **No full solution dump in coaching mode**: the value is in the questions asked, not the answer given.

References

• [Common Designs](references/common-designs.md) - per-problem sketch with constraints, components, and trade-offs